Fingerprinting is one of the most widely used biometric methods for identifying and authenticating individual persons. The modem science of fingerprinting started in the second half of the 19th century. There are two types of fingerprint data, distinguished by their formation processes. In forensic science, finger marks left unintentionally by a suspect at a crime scene are referred to as “latent fingerprints,” while fingerprints acquired directly from human fingers using ink or scanners in controlled environments are referred to as “exemplar fingerprints.” Latent fingerprints differ from exemplar fingerprints in that they are very difficult to detect with unaided human vision under most ordinary viewing conditions (hence their names). Latent fingerprints are usually of lower quality, although it is sometimes possible to find high quality fingerprint marks at a crime scene. These latter fingerprints are called “patent fingerprints.” Nonetheless, it is the latent fingerprints that are more common and require greater efforts to render visible. Most techniques employed for this purpose utilize a chemical or physical process that applies some kind of material directly to the surface suspected to bear fingerprints. Once the contrast of the fingerprint mark is sufficiently enhanced by such treatments, the mark is either photographed or “lifted” in order to be permanently archived as evidence. The term “lifting the fingerprint” originates from the oldest, but still widely used fingerprint detection method—powdering—in which the powders applied adhere to the fingerprint material, and then are physically lifted out of the original crime scene object by a sticky tape.
The currently popular latent fingerprint detection and extraction methods used by law enforcement agencies include, but are not limited to, powdering, iodine fuming, ninhydrin and DFO application, silver nitrate development, cyanoacrylate (glue) fuming, gentian violet staining, episcopic coaxial illumination, laser excited luminescence, and RUVIS (Reflected Ultra Violet Imaging System). Despite these options, there is still a need for new methods because every existing method tends to be unsuitable for some surfaces, due either to its inadequacy in lifting the print from, or to its damaging side effects to the surfaces. In particular, the chemical and physical processing involved to extract latent fingerprints can inflict deleterious effects upon the objects being examined. Thus, often valuable and irreplaceable objects cannot be searched for fingerprints at all. Furthermore, the chemicals used to enhance fingerprint contrast or to induce luminescence may need long processing time, are often toxic, environmentally unfriendly, or even radioactive. Several ingredients used in dusting powders are also known to be toxic or posing potential health hazards, e.g. titanium oxide and manganese dioxide. They can be harmful to the operator if not handled correctly because they are designed to react with or adhere to the fingerprint residues, which are the same material found on human skin. Some chemicals require specific solvents that have undesirable side effects (e.g., methanol (in DFO solution) and phenol (in Gentian violet solution) are poisonous); indeed, some solvents used for fingerprint enhancement have actually been banned because of their damage to the environment, e.g. solvent HFE7100.
There have been a two attempts to develop non-contact latent fingerprint detection and lifting methods, but so far each of them has had its specific shortcomings. The episcopic coaxial illumination method uses a semitransparent mirror to project the light source at a right angle to the surface and to observe the surface also from the right angle. However, this arrangement cannot allow adjustment in lighting angle and observation angles, and both are known to greatly affect the resulting contrast of fingerprint patterns. In addition, the reflection seen at a right angle does not show polarization contrast which the method of the present invention utilizes to further enhance the visibility of latent fingerprints.
Lasers have provided an optical method for lifting latent prints, utilizing induced luminescence of the fingerprint material. However, there are limitations to this method as well. In its original non-contact form the natural fluorescence signal is often very weak, thus requiring a very powerful laser at blue/green wavelength. These powerful lasers are very expensive and generally not portable due to bulky size of the power and cooling requirements. More portable lasers and arc lamps have been found useful to substitute for the fluorescence light source only when used with chemical florescence enhancers (and are thus invasive). Also, the laser must operate in the near ultraviolet and have adequate rating. Such laser equipment is fairly expensive and requires trained technicians to be operated safely. The fact that all organic substances can fluoresce when excited by a laser also causes significant background noise. In addition, many commonly found surfaces, e.g. several kinds of paper, will fluoresce even stronger than the fingerprint residue when illuminated by laser. Thus, laser-excited luminescence, like other existing methods, can not be applied to certain types of surfaces and are most often used with the aid of applying fluorescence enhancing chemicals, the use of which negates the non-contact advantage.
RUVIS is another non-contact method that takes advantage of the contrast differences between the fingerprint and the underlying surface. The equipment used is more portable and less expensive than the powerful laser required in the non-contact laser fluorescence method, but the RUVIS equipment is still much more expensive than the ordinary visible band optical equipment used to implement the method of the invention. This method is successful in some cases but not in some other cases. It is found again that pretreatment with fluorescence enhancing chemicals produces better results in many cases. Thus, RUVIS may be more practical when used as an invasive method.
Automated optical fingerprint extraction systems are commercially available. One proposed system utilizes laser light with a polarizer to extract fingerprint images directly from the live human finger. However, most of such systems are designed to take exemplar fingerprints, i.e. from subjects who cooperate with the system during the acquisition of fingerprint images. As explained above, exemplar fingerprint extraction is a very different application field than latent fingerprint extraction. Exemplar fingerprint scanning systems are used primarily for security systems or law enforcement facilities where the users cooperate with the fingerprint extraction devices but would be totally useless for extracting latent fingerprints at a crime scene.
An optical fingerprinting method is desired that extracts high quality latent fingerprints without any invasive chemical or physical contact with the examined object, and requires no cooperation of the subject. Rather than employing extraneous material, a method is needed that takes advantage of the optical properties of the latent fingerprint, which include sweat (salty water), grease, and lipid, all of which are rather transparent dielectric materials, making them difficult to detect under most viewing conditions. The present invention satisfies these needs in the art.